Centrifugal pumps generally comprise an impeller, which is rotatably mounted in a stationary casing, the rotating impeller imparting pressure and kinetic energy to the fluid being pumped, and the stationary casing guiding the fluid to and from the impeller. The fluid is caused to flow in a generally circular direction about the perimeter of the casing by means of a guidevane which directs fluid exiting the impeller to a discharge nozzle of the pump.
In the operation of centrifugal pumps of such nature, the fluid flow is generally effected by the design and size of the impeller, the design and size of the casing and guidevane, the speed at which the impeller rotates, the design and size of the pump inlet and outlet, quality of finish of the components and the like.
In a popular commercially available centrifugal pump arrangement the pump is an assembly of multiple stamped metal parts. A base mounting plate is mounted nearest the motor and comprises a generally central opening through which a shaft from the motor extends for mounting of an impeller. A seal is mounted at the central opening of the base plate between the motor shaft and the base plate to resist reverse flow of fluids from the pump along the shaft.
A generally circular pump casing is mounted and sealed to the base plate, which together therewith enables a sealed housing containing a motor shaft driven impeller assembly, a guidevane (diffuser), inlet and outlet of the pump.
The impeller may comprise a fabricated assembly or may be molded, machined or otherwise formed from a single billet of material. In a particularly popular embodiment, whether molded from a billet or assembled, the impeller can be generally described as comprising opposing circular front and rear impeller plates, which contain therebetween vents which radiate from a central fluid intake opening in the impeller to the periphery of the impeller.
The impeller assembly is mounted to the shaft of the motor to rotate therewith, generally by means of a threaded mount which is positioned along about the center axis of the impeller spaced rearward toward the motor from the rear impeller plate to enable flow of fluid through the central intake opening past the front impeller plate through the radiating vents and out the periphery of the impeller. The front plate of the impeller further comprises a raised cylindrical shoulder (nose) which extends forward from the plate toward the inlet of the casing to enable a conduit for fluid flow from the inlet of the pump casing to the central opening of the impeller.
A guidevane, or diffuser, is generally press fit mounted to the interior of the casing and generally divides the casing among a rearward impeller section comprising the area from the base mounting plate to the guidevane and a forward outlet section comprising the area from the guidevane to the outlet. The guidevane generally comprises a dished circular plate with tangentially streamlined flow directors comprising openings along its perimeter. The flow directors are arranged to extend over the periphery of the impeller and direct fluid passing through the vanes of the rotating impeller, into the forward outlet section of the casing, in a stream circulating around the rounded interior of the wall of the casing. The design and placement of the guidevane is critical to achieving and maintaining a particular flow design of the circulating pump and constitutes an operational seal among the differential pressures between the impeller section and the outlet section.
The guidevane has an impeller opening to enable it to be mounted over an inlet conduit. The inlet conduit provides influent to the central opening of the impeller and generally comprises a conduit means extending from an opening in the casing of the pump to the cylindrical shoulder/nose of the impeller arranged about the impeller's central opening. Fluids in the inlet conduit are generally under a negative pressure in that the impeller sucks fluid therein. The impeller opening of the guidevane is critically sized to enable rotation of the shoulder of the forward plate of the impeller without engaging the edge of the impeller opening of the guidevane, but to a close tolerance to maintain an operational seal between the impeller section and the outlet section and may even include an operational seal among the inlet conduit, the outlet section and/or the impeller section.
Pumps of the above arrangement have manufacturing and assembly advantages in that the components can be conveniently produced from stamped thin metals and easily assembled with minimum skill. However, such arrangement has been found to have inherent failure problem. As the guidevane is stationary and the impeller rotating, the operational seal between the components typically involves maintaining a critical close tolerance impeller opening which enables impeller rotation without significant flow of fluids through the impeller opening between the guidevane and the shoulder of the impeller. Typical low cost metal stamping processes make it difficult to maintain close tolerances. Since the efficiency of the pump generally requires the maintenance of differential pressures at the operational seal, factors which may contribute to degradation of the operational seal may effect efficiency and pump life, involving costly down time and repair costs.
Fluids pumped in such arrangements characteristically are subject to significant lamellar flow sheering forces along the shoulder of the impeller forward plate particularly at about the critical operational seals and thus, can exacerbate degradation of operational seals. In utilities wherein particulate containing materials and/or fluid mixtures are being pumped, the lamellar sheering forces can degrade the material comprising the impeller and/or the edge of the impeller opening of the guidevane, beyond the design limits of the operational seal.
Various means have been tried to prevent the failure of operational seals. One means is to mount a metal or the like "O" ring seal on the collar of the impeller and maintain a critical tolerance among it and the guidevane impeller opening. However, such seals tend to be loose or loosen from the collar during operation of the pump and upon loosening have been known to cut through the collar and create further repair problems. In addition, the thin metals used in popular guidevanes can flex and otherwise distort during operation so that critical tolerances can be elusive. No satisfactory method of resolving the problem appears to have been found.
It is an object of the present invention to provide an improved seal among the critical tolerance at the guidevane impeller opening and the impeller shoulder.
It is a further object of the invention to provide an improved circular pump having longer wear characteristics.
These and other objects of the invention will become apparent from the following.